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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Incommensurate Spin Fluctuations in the Spin-Triplet Superconductor Candidate UTe 2

Spin-triplet superconductors are of extensive current interest because they can host topological state and Majorana fermions important for quantum computation. The uranium-based heavy-fermion superconductor UTe 2 has been argued as a spin-triplet superconductor similar to UGe2, URhGe, and UCoGe, where the superconducting phase is near (or coexists with) a ferromagnetic (FM) instability and spin-triplet electron pairing is driven by FM spin fluctuations. Here we use neutron scattering to show that, although UTe 2 exhibits no static magnetic order down to 0.3 K, its magnetism in the [0,K,L] plane is dominated by incommensurate spin fluctuations near an antiferromagnetic ordering wave vector and extends to at least 2.6 meV. We are able to understand the dominant incommensurate spin fluctuations of UTe 2 in terms of its electronic structure calculated using a combined density-functional and dynamic mean-field theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Spatiotemporal mapping of mesoscopic liquid dynamics

The study of liquid dynamics at mesoscopic scales is still strewn with difficulty due to limitations in theory and experiment. Historically, significant attention has been given to the analysis of space-time correlation functions and their frequency-Fourier transforms at a few discrete wave numbers. The massive computing power afforded by modern high performance computing clusters and the advent of a wide-angle neutron spin-echo spectrometer, however, have unlocked a more intuitive and fruitful approach to this problem. Using molecular dynamics simulations, here we demonstrate the benefits of spatiotemporally mapping intermediate scattering functions on a dense grid of correlation times and wave numbers. Four model systems are investigated: a Lennard-Jones liquid, a coarse-grained bead-spring polymer, a molten sodium chloride, and a poly(ethylene oxide) melt. We show that the spatiotemporal mapping approach is particularly useful for elucidating the mesoscopic dynamics in these liquids, where several underlying mechanisms, such as molecular relaxations, hydrodynamic modes, and nonhydrodynamic excitations, are potentially at play. Therefore, compared to the traditional method, direct visualization of density space-time correlation functions on two-dimensional color maps permits appraisals of complicated dynamical behavior at mesoscales in a global manner. For example, the scaling relations between space and time for different types of molecular motions can be straightforwardly identified on these plots, without any model-dependent analysis. Additionally, we show how theoretical ideas regarding collective mesoscopic dynamics, such as the classical hydrodynamic theory, the convolution approximation, and a recently proposed phenomenological model, can be discussed in terms of the global features of spatiotemporal maps of intermediate scattering functions. The new perspective offered by the spatiotemporal mapping method should prove useful for the study of liquid dynamics in general.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Electronic properties of ThC − and ThC

The present study investigates the properties of low-lying states of ThC − and ThC using correlated wave function theories. To this end, we employed multireference calculations and various coupled-cluster approaches in combination with large correlation-consistent basis sets. These methods were applied to examine potential energy curves (PECs), electron configurations, energetics, spectroscopic constants, and spin–orbit coupling effects for 9 states of ThC − and 18 states of ThC. The ground states of ThC − and ThC were identified as single-reference 2 Σ + 1/2 (I) and $^3Σ^+_{0^-} (\textrm{I})$, respectively. Electron detachment from the 7s orbital of the Th center in ThC − [ 2 Σ + (I); 1σ 2 2σ1 3 σ2 1 π 4 ] yields ThC [ 3 Σ + (I); 1σ 2 2σ 1 3σ 1 1π 4 ]. The calculated adiabatic detachment energy (ADE) and vertical detachment energy (VDE) for this process are 1.591 and 1.604 eV, respectively. Furthermore, the dissociation energy (D 0 ) of ThC [$^3Σ^+_{0^-} (\textrm{I})$] is predicted to be 5.099 eV. The standard enthalpy of formation, $ΔH^{°}_{\textrm{f}}$ (298 K), of ThC is estimated to be 822.52 ± 6 kJ mol −1 .

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS2004

Space-Based Gravitational-Wave Observations as Tools for Testing General Relativity

We continued a project, to analyse the ways in which detection and study of gravitational waves could provide quantitative tests of general relativity, with particular emphasis on waves that would be detectable by space-based observatories, such as LISA. This work had three foci: 1) Tests of scalar-tensor theories of gravity that, could be done by analyzing gravitational waves from neutron stars inspiralling into massive black holes, as detectable by LISA; 2) Study of alternative theories of gravity in which the graviton could be massive, and of how gravitational-wave observations by space-based detectors, solar-system tests, and cosmological observations could constrain such theories; and 3) Study of gravitational-radiation back reaction of particles orbiting black holes in general relativity, with emphasis on the effects of spin.

Will, Clifford M.↗
DOE OSTI2023

Collective dynamics of polarized spin-half fermions in relativistic heavy-ion collisions

Standard relativistic hydrodynamics has been successful in describing the properties of the strongly interacting matter produced in the heavy-ion collision experiments. Recently, there has been a significant theoretical advancement in this field to explain spin polarization of hadrons emitted in these processes. Although current models have successfully explained some of the experimental data based on the coupling between spin polarization and vorticity of the medium, they still lack a clear understanding of the differential measurements. This is commonly interpreted as an indication that the spin needs to be treated as an independent degree of freedom whose dynamics is not entirely bound to flow circulation. In particular, if the spin is a macroscopic property of the system, in equilibrium its dynamics should follow hydrodynamic laws. Here, we develop a framework of relativistic hydrodynamics which includes spin degrees of freedom from the quantum kinetic theory for Dirac fermions and use it for modeling the dynamics of matter. Following experimental observations, we assume that the polarization effects are small and derive conservation laws for the net baryon current, the energy–momentum tensor and the spin tensor based on the de Groot–van Leeuwen–van Weert definitions of these currents. We present various properties of the spin polarization tensor and its components, analyze the propagation properties of the spin polarization components, and derive the spin-wave velocity for arbitrary statistics. We find that only the transverse spin components propagate, analogously to the electromagnetic waves. Finally, using our framework, we study the space–time evolution of the spin polarization for the systems respecting certain space–time symmetries and calculate the mean spin polarization per particle, which can be compared to the experimental data. We find that, for some observables, our spin polarization results agree qualitatively with the experimental findings and other model calculations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Search for nonreciprocal magnons in MnPS 3

Recent articles have suggested that the quasi-two-dimensional antiferromagnet MnPS 3 may have nonreciprocal magnons, whereby magnons in a Brillouin zone corner at + q have different energies than those at q . The magnons along the Brillouin zone boundaries were measured using neutron three-axis spectrometry, paying careful attention to the resolution function, to determine whether such nonreciprocity was present. The data show that, within the resolution, there are no significant differences between the magnons in opposite Brillouin zone corners. The findings are combined with a recently published study of MnPS 3 by neutron resonant spin-echo spectroscopy and discussed within the context of the theories for nonreciprocal magnons.

2-dimensional systems↗
DOE OSTI2025

Isostructural Phase Transition of Fe 2 ⁢O 3 under Laser Shock Compression

We present in situ x-ray diffraction and velocity measurements of Fe 2⁢ O 3 under laser shock compression at pressures between 38–122 GPa. None of the high-pressure phases reported by static compression studies were observed. Instead, we observed an isostructural phase transition from 𝛼−Fe 2⁢ O 3 to a new 𝛼′−Fe 2 ⁢O 3 phase at a pressure of 50–62 GPa. The 𝛼′−Fe 2⁢ O 3 phase differs from 𝛼−Fe 2 ⁢O 3 by an 11% volume drop and a different unit cell compressibility. We further observed a two-wave structure in the velocity profile, which can be related to an intermediate regime where both 𝛼 and 𝛼′ phases coexist. Furthermore, density functional theory calculations with a Hubbard parameter indicate that the observed unit cell volume drop can be associated with a spin transition following a magnetic collapse.

36 MATERIALS SCIENCE↗
DOE OSTI2020

First-principles study of the low-temperature charge density wave phase in the quasi-one-dimensional Weyl chiral compound ( TaSe 4 ) 2 I

Using ab initio density functional theory, we study herein the lattice phase transition of quasi-one-dimensional (TaSe 4 ) 2 I. In the undistorted state, the strongly anisotropic semimetal band structure presents two nonequivalent Weyl points. In previous efforts, two possible Ta-tetramerization patterns were proposed to be associated with the low-temperature structure. Our phonon calculations indicate that the orthorhombic F222 CDW-I phase is the most likely ground state for this quasi-one-dimensional system. In addition, the monoclinic C2 CDW-II phase may also be stable according to the phonon dispersion spectrum. Since these two phases have very similar energies in our density functional theory calculations, both these Ta-tetramerization distortions likely compete or coexist at low temperatures. The semimetal-to-insulator transition is induced by a Fermi-surface-driven instability that supports the Peierls scenario, which affects the Weyl physics developed above T CDW . Furthermore, the spin-orbit coupling generates Rashba-like band splittings in the insulating charge density wave phases.

36 MATERIALS SCIENCE↗
NASA NTRS1979

Studies of magnetostriction and spin polarized band structures of rare earth intermetallics

Anisotropic magnetostriction measurements of R6Fe23, R = (Tb, Dy, Ho, and Er) were carried out from 77 K to room temperature. Magnetic fields up to 2.1 Tesla were applied. All the compounds exhibited large magnetostrictions at 77 K, the largest effect being obtained for Tb6Fe23. Saturation magnetostriction values for the compounds were also determined for 77 K and room temperature. Results of the temperature dependence of magnetostriction for Er6Fe23 are in good agreement with Callen and Callen's single ion theory. Therefore, the main sources of magnetostriction in this compound is the Er ion. The spin-up and spin-down electronic energy bands, the density of states and the magnetic moments of YCo5, SmCo5, and GdCo5 were calculated by the spin polarized augmented plane wave technique. The calculations obtained show the origin of the moment, provide good estimates of its magnitude and variation, and the reasons for those variations. They also show the important role of partial charge transfer and of d-d electronic coupling. Calculations for LaNi5 and GdNi5 systems are discussed.

Wallace, W. E.↗
DOE OSTI2024

Structural phase transition, s ± -wave pairing, and magnetic stripe order in bilayered superconductor La 3 Ni 2 O 7 under pressure

Motivated by the recently discovered high-T c superconductor La 3 Ni 2 O 7 , we comprehensively study this system using density functional theory and random phase approximation calculations. At low pressures, the Amam phase is stable, containing the Y 2- mode distortion from the Fmmm phase, while the Fmmm phase is unstable. Because of small differences in enthalpy and a considerable Y 2- mode amplitude, the two phases may coexist in the range between 10.6 and 14 GPa, beyond which the Fmmm phase dominates. In addition, the magnetic stripe-type spin order with wavevector (π, 0) was stable at the intermediate region. Pairing is induced in the s ± -wave channel due to partial nesting between the M = (π, π) centered pockets and portions of the Fermi surface centered at the X = (π, 0) and Y = (0, π) points. This resembles results for iron-based superconductors but has a fundamental difference with iron pnictides and selenides. Moreover, our present efforts also suggest La 3 Ni 2 O 7 is qualitatively different from infinite-layer nickelates and cuprate superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Leading-order gravitational radiation to all spin orders

Starting with on-shell amplitudes compatible with the scattering of Kerr black holes up to Compton-amplitude contact terms, we produce the gravitational waveform and memory effect including spin at their leading post-Minkowskian orders to all orders in the spins of both scattering objects. For the memory effect, we present results at next-to-leading order as well, finding a closed form for all spin orders when the spins are anti-aligned and equal in magnitude. Considering instead generically oriented spins, we produce the next-to-leading-order memory to sixth order in spin. Compton-amplitude contact terms up to sixth order in spin are included throughout our analysis.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Quantum embedding for material chemistry based on domain separation and open subsystems

Our perspective considers two theories we recently proposed to perform quantum embedding calculations for chemical systems: domain-separated density functional theory (DSDFT) and locally coupled open subsystems (LCOS). The development includes both the fundamentals of each theory as well as potential applications, some technical aspects, and related challenges. DS-DFT is suited to study intramolecular effects, where one can apply a high level of theory (based on DFT or wave function theory) to a region of interest inside a molecule or solid, and lower level theory elsewhere, with smooth switching between the regions. LCOS, in contrast, is a fragment-based embedding which offers computational advantages to study intermolecular behavior such as electron hopping, spin-environment interaction, and charge transfer excitations. Yet, both theories can exchange roles when appropriate. In addition, these theories allow for control of computational scaling of their algorithms. We investigate paths to determine the charge-transfer operator used in LCOS, and suggest an auxiliary energy minimization that can provide a practical estimate to this operator. We also briefly discuss how to implement density fitting techniques in domain separation, and how domain separation can be used for pure wavefunction-based embedding.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1972

An investigation of compressor noise by use of a spinning mode synthesizer.

Use of the Spinning Mode Synthesizer (SMS) as a tool for studying the properties of such complex waves as those generated by fan engines. This has greatly reduced the amount of trial and error techniques used for optimizing duct liners. The SMS was designed by using simple transducer theory and properly controlling the phasing to each of 8 loudspeakers. It provides a means for making in-duct measurements of the radial, axial, and circumferential modal distributions. Results are presented which show that the device is of value for investigation of the highly complex nature of compressor radiated noise.

Oslac, M. J.↗
DOE OSTI2021

Interpretations of ground-state symmetry breaking and strong correlation in wavefunction and density functional theories

Significance The ground state of a quantum mechanical system is the lowest-energy eigenstate of the Hamiltonian. In isolation, it persists unchanged forever, with symmetries dictated by those of the Hamiltonian. But near-eigenstates of broken symmetry can persist for long times, even on the scale of human measurement. The appearance of broken symmetries of the electron density or spin density in a density functional calculation can reveal strong correlations among the electrons that are present in a symmetry-unbroken wavefunction. Symmetry breaking can arise when a wave-like fluctuation drops to zero frequency. The presented examples are the stretched hydrogen molecule, antiferromagnetism in solids, and the static charge-density wave in a low-density jellium, which is shown quantitatively to be a zero-frequency plasma wave.

42 ENGINEERING↗
NASA NTRS1974

Spin projection of single-determinant wavefunctions

The components of the one- and two-particle density matrices resulting from spin projection of a single-determinant wave function are rederived by the method of expansion in terms of the natural orbitals of charge density, and the results are found to diverge from those given by Harriman and Sando for the two-particle case. The theory is generalized to include molecules with unequal numbers of electrons and basis orbitals and is applied to a number of organic molecules and ions. The correctness and internal consistency of the results argue in favor of the modification described.

Phillips, D. H.↗
NASA NTRS2021

Tests of General Relativity with Binary Black Holes from the second LIGO–Virgo Gravitational-Wave Transient Catalog

Gravitational waves enable tests of general relativity in the highly dynamical and strong-field regime. Using events detected by LIGO-Virgo up to 1 October 2019, we evaluate the consistency of the data with predictions from the theory. We first establish that residuals from the best-fit waveform are consistent with detector noise, and that the low- and high-frequency parts of the signals are in agreement. We then consider parametrized modifications to the waveform by varying post-Newtonian and phenomenological coefficients, improving past constraints by factors of ∼2; we also find consistency with Kerr black holes when we specifically target signatures of the spin-induced quadrupole moment. Looking for gravitational-wave dispersion, we tighten constraints on Lorentz-violating coefficients by a factor of ∼2.6 and bound the mass of the graviton to m(g)≤1.76 x 10^(-23) eV/sq. c with 90% credibility. We also analyze the properties of the merger remnants by measuring ringdown frequencies and damping times, constraining fractional deviations away from the Kerr frequency to δ{\hat {f}}(220)=0.03(+0.38,-0.35) for the fundamental quadrupolar mode, and δ{\hat {f}}(221)=0.04(+0.27,-0.32) for the first overtone; additionally, we find no evidence for postmerger echoes. Finally, we determine that our data are consistent with tensorial polarizations through a template-independent method. When possible, we assess the validity of general relativity based on collections of events analyzed jointly. We find no evidence for new physics beyond general relativity, for black hole mimickers, or for any unaccounted systematics.

R. Abbott↗
DOE OSTI2026

Kramers nodal line in the charge density wave state of YTe 3 and the influence of twin domains

Recent studies have focused on the relationship between charge density wave (CDW) collective electronic ground states and nontrivial topological states. YTe 3 , a nonmagnetic quasi-two-dimensional chalcogenide, has been reported to exhibit a CDW state below 334 K. Using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), we establish that YTe 3 is a CDW-induced Kramers nodal line (KNL) metal, a recently proposed topological state of matter. Scanning tunneling microscopy and low energy electron diffraction reveal two orthogonal domains, each with a unidirectional CDW and a similar wave vector (𝐪 CDW ). When the influence of twin domains is considered, the effective band structure (EBS) computations that utilize DFT-calculated bands using a noncentrosymmetric structure determined by x-ray crystallography, show excellent agreement with ARPES. The noncentrosymmetry of YTe 3 is established by Raman spectroscopy. The Fermi surface and ARPES intensity plots show weak shadow bands displaced by 𝐪 CDW from the main bands. Furthermore, these are linked to CDW modulation, as the EBS calculation confirms. Bilayer split main and shadow bands suggest the existence of crossings, according to theory and experiment. DFT bands, including spin-orbit coupling, indicate existence of a KNL along the Σ direction from multiple crossings of bands dispersing perpendicular to it. Additionally, doubly degenerate bands are only found along the KNL at all energies, with some bands dispersing through the Fermi level.

Angle-resolved photoemission spectroscopy↗